Solid state imaging device, driving method of the solid state imaging device and electronic equipment

ABSTRACT

A solid state imaging device including multiple unit pixels including a photoelectric converter generating electrical charge in accordance with incident light quantity and accumulating the charge, a first transfer gate transferring the accumulated charge, a charge holding region holding the transferred charge, a second transfer gate transferring the held charge, and a floating diffusion region converting the transferred charge into voltage; an intermediate charge transfer unit transferring, to the charge holding region, a charge exceeding a predetermined charge amount as a first signal charge; and a pixel driving unit setting the first transfer gate to a non-conducting state, set the second transfer gate to a conducting state, transfer the first signal charge to the floating diffusion region, set the second transfer gate to a non-conducting state, set the first transfer gate to a conducting state, and transfer the accumulated charge to the charge holding region as a second signal charge.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.13/603,633, filed Sep. 5, 2012, which is a continuation of U.S. patentapplication Ser. No. 12/411,954, filed Mar. 26, 2009, the entireties ofwhich are incorporated herein by reference to the extent permitted bylaw. The present application claims the benefit of priority to JapanesePatent Application No. JP 2008-096884 filed in the Japan Patent Officeon Apr. 3, 2008, and Japanese Patent Application No. JP 2009-075169filed Mar. 25, 2009, the entireties of which are incorporated byreference herein to the extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid state imaging device, a drivingmethod of the solid state imaging device, and electronic equipment.

2. Description of the Related Art

An example of a solid-state imaging device is a CMOS image sensor whichreads out photo-generated charges accumulated in the p-n junctioncapacitance of photodiodes, which are photoelectric conversion devices,by way of MOS transistors. With such a CMOS image sensor, actions forreading out photo-generated charges accumulated in photodiodes areexecuted for each pixel, each line, or the like. Accordingly, theexposure period for accumulating photo-generating charge does not matchfor all pixels, and distortion occurs in the image when the subject ismoving, etc.

FIG. 38 illustrates a structural example of a unit pixel (hereinaftermay be referred to simply as “pixel”). As shown in FIG. 38, the unitpixel 100 is of a configuration including, in addition to a photodiode101, also a transfer gate 102, an n-type floating diffusion (FD) 103, areset transistor 104, an amplifying transistor 105, and a selectingtransistor 106.

With this unit pixel 100, the photodiode 101 is an embedded photodiodewherein a p-type layer 113 has been formed on a p-type well layer 112formed on an n-type substrate 111 and an n-type embedded layer 114embedded. The transfer gate 102 transfers charges accumulated at the p-njunction of the photodiode 101 to the floating diffusion 103.

Mechanical Shutter Method

One widely-used way of realizing global exposure, which is anarrangement wherein imaging is performed with all pixels exposed at thesame exposure period, with the solid state device having theabove-described unit pixel 100, is the mechanical shutter method whichuses mechanical light shielding. Exposure is started for all pixels atthe same time, and exposure is ended for all pixels at the same time,whereby global exposure is realized.

With the mechanical shutter method, the period during which light isinput to the photodiode 101 and photo-generated charge is generated isthe same for all pixels, due to mechanically controlling the exposuretime. With this system, the mechanical shutter closes so substantiallyno more photo-generated charges are generated, and in this state signalsare sequentially read out. However, reduction in size is difficult sincea mechanical shielding mechanism is used, and also the mechanicalshutter method is inferior to electrical methods with regard tosimultaneity as well, due to limitation of the mechanical driving speed.

Global Exposure According to the Related Art

Operations for realizing imaging with the exposure period matched forall pixels and with no distortion using the unit pixel 100 shown in FIG.38 will be described with reference to the operation explanatory diagramin FIG. 39 and the timing chart in FIG. 40.

First, a discharging operation for emptying the accumulated charges inthe embedded photodiodes 101 of all pixels at the same time is executed,and exposure is started (1). Thus, a photo-generated charge isaccumulated at the p-n junction of the photodiode 101 (2). At the pointthat the exposure period has ended, the transfer gate 102 is turned onfor all pixels at the same time, so all of the accumulated charges aretransferred to the floating diffusion (capacitance) 103 (3). Closing thetransfer gate 102 holds the photo-generated charges accumulated over thesame exposure period for all pixels in the respective floatingdiffusions 103. Subsequently, the signal levels are sequentially readout to a vertical signal line 200 (4), following which the floatingdiffusion 103 is reset (5), and after this the reset level is read outto the vertical signal line 200 (6).

After having read out the signal level and reset level to the verticalsignal line 200, noise removal processing of the signal level isperformed using the reset level, in downstream signal processing. Withthis noise removal processing, the reset level of the reset operationexecuted after reading out the signal level is read out, so kTC noiseoccurring in the reset operation is not removed, which can lead to imagedeterioration.

The kTC noise occurring in the reset operation is random noise generatedby the switching operations of the reset transistor 104 at the time ofthe reset operation, so the signal level noise will not be preciselyremoved unless using the level before transfer of the charge to thefloating diffusion 103. The charge is transferred to the floatingdiffusion 103 at the same time for all pixels, so the reset operation isperformed again following reading out the signal level, and noiseremoval is performed. Accordingly, noise such as offset error and thelike can be removed, but this is not the case with kTC noise.

Now, we will refer the readout period of the signal level as “D period”,and the readout period of the reset level as “P period”. There are manycrystal flaws at the Si—SiO2 interface, and dark current readily occurs.In the event of holding that charge at the floating diffusion 103, thereis difference which occurs in the dark current applied to the signallevel, depending on the readout order. This also is not cancelable withnoise removal using the reset level.

Pixel Structure Having Memory Unit

One proposal which has been made as a way to deal with the problem ofthe above-described kTC noise not being removable is a unit pixel 300which has a memory unit (MEM) 107 separately from the floating diffusion103 within the pixel (e.g., see Japanese Unexamined Patent ApplicationPublication (Translation of PCT Application) No. 2007-502722 andJapanese Unexamined Patent Application Publication No. 2006-311515) asshown in FIG. 41. The memory unit 107 temporarily holds thephoto-generated charge accumulated at the embedded photodiode 101. Theunit pixel 300 further is provided with a transfer gate for transmittingthe photo-generated charge accumulated at the photodiode 101 to thememory unit 107.

The operations for executing global exposure at the unit pixel 300having the memory unit 107 will be described with reference to theoperation explanatory diagram in FIG. 42.

First, a discharging operation is executed for all pixels at the sametime, and exposure is started (1). A photo-generated charge isaccumulated at the photodiode 101 (2). At the time of ending theexposure, the transfer gate 108 is driven for all pixels at the sametime to transfer the photo-generated charge to the memory unit 107,where it is held (3). Following exposure, the reset level and signallevel are read out in sequential operations.

First, the floating diffusion 103 is reset (4), and next the reset levelis read out (5). Subsequently, the charge held at the memory unit 107 istransferred to the floating diffusion 103, and the signal level is readout (7). At this time, the reset noise included in the signal levelmatches the reset noise read out in the reset level readout, so noisereduction processing including the kTC noise as well can be performed.

As can be understood from the above description, a pixel structurehaving the memory unit 107 for temporarily holding the photo-generatedcharge accumulated at the embedded photodiode 101 can realize noisereduction processing including the kTC noise as well.

SUMMARY OF THE INVENTION

However, in the case of the pixel structure disclosed in JapaneseUnexamined Patent Application Publication No. 2007-502722, when comparedto a photoreceptor (photodiode) not having the memory unit 107, the areaof the photoreceptor is reduced by an amount equivalent to that of thememory unit 107 if the pixel size is the same, so the saturation chargeQs_pd is lower. A lower saturation charge Qs_pd means a lower dynamicrange. Even if global exposure is realized, reduction in the dynamicrange, which is an important property of a camera set, will result inmarked deterioration in photographed image quality.

On the other hand, the pixel structure described in Japanese UnexaminedPatent Application Publication No. 2006-311515 prevents drop in dynamicrange by widening by luminosity level which can be handled by way oflogarithmic response. The details thereof are as follows.

In the pixel structure shown in FIG. 41, we will say that a voltagevalue which is conduced through the transistors is a first voltagevalue, a non-conducting voltage value is a second voltage value, and anintermediate voltage value is a third voltage value. At the time ofperforming an imaging operation at the photoelectric converter, thetransfer gate 102 is in a conducting state, and the third voltage valueis applied to the transfer gate 108. Accordingly, the transfer gate 108operates at a sub-threshold region with regard to at least a partialluminosity range of the incident light to the embedded photodiode 101.

FIG. 43 illustrates the operating state of the above-describedlogarithmic response operation. As described above, in order to have alogarithmic response operation, the transfer gate 102 has to be in aconducting state, and further the reset transistor 104 also has to be ina conducting state, so as to form a path from the reset voltage VDB forthe photocurrent Iph. FIG. 44 shows an equivalent circuit at the time oflogarithmic response operation.

The photocurrent Iph proportionate to the incident light luminosity Eflows to the transfer gate 108 to which the intermediate voltage (thirdvoltage value) has been applied, so the transfer gate 108 operates at asub-threshold region. Accordingly, the source/drain voltage Vdrop is

Vdrop∝log(Iph)

as to the photocurrent Iph.

Consequently, the potential of the photodiode 101 is VDB−Vdrop. If wesay that the potential of the photodiode 101 when there is noaccumulation of photoelectric charge is VPD, the accumulated charge Qcan be obtained by

Q=Cpd·{VPD−(VDB−Vdrop)}

=Cpd·{VPD−(VDB−α·log(Iph)+β)}

where Cpd represents the parasitic capacitance of the photodiode 101,and α and β are constants determined by the threshold of the transfergate 108 and so forth.

That is to say, the accumulated charge Q is not accumulatedproportionately to the incident light luminosity E; rather, a chargeequivalent to a voltage value having a logarithmic relation remaining inthe photodiode 101. FIG. 45 shows the relation between the incidentlight luminosity E and the pixel output. As can be clearly understoodfrom FIG. 45, the response is linear up to the point of switching overto logarithmic response, and follows a logarithm function afterexceeding a certain luminosity level E0.

Now, E0 is determined by the intermediate voltage (third voltage value)and the threshold value of the transfer gate 108. Accordingly, if thereare irregularities in threshold values of the transfer gates 108, theswitchover point E0 differs from one pixel to another as shown in FIG.46, causing marked irregularity in the input/output properties of thepixels. This causes deterioration in image quality as fixed patternnoise. Also, the potential of the photodiode 101 is determined in thestate with the current applied, leading to noise such as thermal noise.

There has been found demand to provide a solid state imaging deviceenabling reduction of noise owing to threshold irregularities in pixeltransistors (transfer gates) among pixels with reduction in saturationcharge amount suppressed, and to provide a driving method of the solidstate imaging device and electronic equipment incorporating the solidstate imaging device.

According to an embodiment of the present invention, a solid stateimaging device includes: a plurality of unit pixels including aphotoelectric converter configured to generate electrical charge inaccordance with incident light quantity and accumulate the chargetherein, a first transfer gate configured to transfer the chargeaccumulated in the photoelectric converter, a charge holding regionconfigured to hold the charge transferred from the photoelectricconverter by the first transfer gate, a second transfer gate configuredto transfer the charge held in the charge holding region, and a floatingdiffusion region configured to hold the charge, transferred from thecharge holding region by the second transfer gate, for being read out asa signal (convert into voltage); an intermediate charge transfer unitconfigured to transfer, to the charge holding region, a charge whichexceeds a predetermined charge amount, generated at the photoelectricconverter in an exposure period in which all of the plurality of unitpixels perform imaging operations at the same time, as a first signalcharge; and a pixel driving unit configured to, in the exposure periodin which all of the plurality of unit pixels perform imaging operationsat the same time, set the first transfer gate to a non-conducting state,set the second transfer gate to a conducting state, transfer the firstsignal charge from the charge holding region to the floating diffusionregion, set the second transfer gate to a non-conducting state, set thefirst transfer gate to a conducting state, and transfer the chargeaccumulated in the photoelectric converter to the charge holding regionas a second signal charge; wherein the pixel driving unit reads out thefirst signal charge as a first output signal in increments of singlepixels or increments of a plurality of pixels after the exposure periodhas ended, then resets the floating diffusion region and reads out thereset level of the floating diffusion region as a reset signal, thensets the second transfer gate to a conducting state and transfers thesecond signal charge to the floating diffusion region, and subsequentlyreads out the second signal charge as a second output signal.

The photo-generated charge generated by photoelectric conversion at thephotoelectric converter is held in the photoelectric converter when theilluminance is low and the charge is at or below a predetermined charge.In the event that the illuminance is high and the charge exceeds thepredetermined charge amount, the portion thereof which exceeds thepredetermined charge amount is transferred to the charge holding regionas a first signal charge. Accordingly, the photo-generated chargegenerated by photoelectric conversion is divided and accumulated as afirst signal charge in the charge holding region and a second signalcharge in the photoelectric converter. Now, while irregularities in thethreshold value of the first transfer gate which is a pixel transistordoes affect the charge being accumulated in the charge holding region,there is no effect on the final unit pixel input/output properties. Forexample, let us say that that the entire charge amount at a certainpixel is divided and accumulated as the first signal charge and secondsignal charge, while at another pixel there was no transfer to the firstsignal charge by ΔQth due to irregularities in threshold value. However,even in this case, the accumulation at the photoelectric converter isthe second signal charge+ΔQth, which the accumulation at the chargeholding region is the first signal charge−ΔQth. Here, the output of theunit pixel is the sum of the first signal charge and the second signalcharge, so the deviation (increase/decrease) ΔQth of the accumulatedcharge due to irregularities in the threshold value of the firsttransfer gate is ultimately cancelled out. Consequently, noise due toirregularities in the threshold value of pixel transistors among thepixels can be reduced.

According to the above configuration, noise due to irregularities in thethreshold value of pixel transistors among the pixels can be reduced, soimage quality of imaged images can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system configuration diagram schematically illustrating theconfiguration of a CMOS image sensor to which an embodiment of thepresent invention is applied;

FIG. 2 is a diagram illustrating the configuration of a unit pixelaccording to a first structural example;

FIG. 3 is a diagram illustrating the configuration of a unit pixelaccording to a modification of the first structural example;

FIG. 4 is an operation explanatory diagram of a case wherein adischarging unit is not used;

FIG. 5 is a potential diagram in the Z-Z′ direction shown in FIG. 2;

FIG. 6 is a diagram exemplarily illustrating the relation between gatevoltage and surface potential;

FIG. 7 is a diagram illustrating the configuration of a unit pixelaccording to a second structural example;

FIG. 8 is a diagram illustrating the configuration of a unit pixelaccording to a third structural example;

FIG. 9 is a potential diagram illustrating the potential at each portionof the unit pixels according to the first through third structuralexamples;

FIGS. 10A through 10E are operation explanatory diagrams of firstthrough fifth features of the embodiment;

FIG. 11 is a timing chart for explanation of operations performed innormal global exposure operations;

FIG. 12 is an operation explanatory diagram of normal global exposureoperations;

FIG. 13 is a timing chart illustrating the driving timing in a case ofusing the unit pixel according to the first structural example;

FIG. 14 is an operation explanatory diagram illustrating transfer ofphoto-generated charge in a case where the incident light in theaccumulation period is strong;

FIG. 15 is an operation explanatory diagram illustrating transfer ofphoto-generated charge in a case where the incident light in theaccumulation period is weak;

FIG. 16 is an operation explanatory diagram illustrating operations inthe readout period;

FIGS. 17A and 17B are diagrams illustrating driving timing for globalexposure, wherein FIG. 15A illustrates a case of normal global exposure,and FIG. 15B illustrates a case of global exposure according to theembodiment;

FIG. 18 is a diagram illustrating another driving timing for globalexposure according to the embodiment;

FIG. 19 is a diagram illustrating the configuration of the unit pixelaccording to the fourth structural example;

FIG. 20A is a potential diagram illustrating X-direction potential inFIG. 19;

FIG. 20B is a potential diagram illustrating Z-direction potential inFIG. 19;

FIG. 21 is a diagram illustrating another configuration of a unit pixelaccording to the fourth structural example;

FIG. 22 is a diagram illustrating an overflow path portion shown in FIG.19;

FIGS. 23A through 23C are plan views illustrating the planar structureof a unit pixel;

FIG. 24 is a timing chart illustrating the driving timing in a case ofusing the unit pixel according to the fourth structural example;

FIG. 25 is a timing chart illustrating an example of driving through adifferent voltage (e.g., 0V) in a transient manner, at the time ofdriving at negative potential (pinning voltage);

FIG. 26 is an operation explanatory diagram of exposure in a case ofusing the unit pixel according to the fourth structural example when theincident light is strong;

FIG. 27 is an operation explanatory diagram of exposure in a case ofusing the unit pixel according to the fourth structural example when theincident light is weak;

FIGS. 28A and 28B are diagrams illustrating other driving timings forglobal exposure in a case of using the unit pixel according to thefourth structural example;

FIGS. 29A through 29C are diagrams illustrating charge accumulation innormal global exposure operations;

FIGS. 30A through 30C are diagrams illustrating charge accumulation inglobal exposure operations according to the embodiment;

FIGS. 31A through 31D are diagrams illustrating input/output propertiesof a unit pixel;

FIG. 32 is a timing chart illustrating a driving example with anexpanded dynamic range;

FIG. 33 is an operation explanatory diagram of operations for anexpanded dynamic range;

FIGS. 34A through 34C are diagrams illustrating input/output propertieswhen operating with an expanded dynamic range;

FIG. 35 is a system configuration diagram schematically illustrating theconfiguration of a CMOS image sensor according to a modification of theembodiment;

FIG. 36 is a system configuration diagram schematically illustrating theconfiguration of a CMOS image sensor according to another modificationof the embodiment;

FIG. 37 is a block diagram illustrating an example of the configurationof an imaging apparatus according to an embodiment of the presentinvention;

FIG. 38 is a diagram illustrating a structural example of a unit pixelaccording to the related art;

FIG. 39 is an operation explanatory diagram of global exposure with aunit pixel according to the related art;

FIG. 40 is a timing chart at the time of performing global exposure witha unit pixel according to the related art;

FIG. 41 is a diagram illustrating the configuration of a unit pixelaccording to the related art having a memory unit;

FIG. 42 is an operation explanatory diagram of global exposure with aunit pixel according to the related art having a memory unit;

FIG. 43 is an operation explanatory diagram illustrating an operationstate of logarithmic response operations;

FIG. 44 is a circuit diagram illustrating an equivalent circuit forlogarithmic response operations;

FIG. 45 is a diagram illustrating the relation between incident lightluminosity E and pixel output; and

FIG. 46 is a diagram illustrating how input/output properties differamong pixels due to irregularities in the threshold values oftransistors among pixels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a detailed description of an embodiment according tothe present invention, with reference to the drawings.

System Configuration

FIG. 1 is a system configuration diagram schematically illustrating theconfiguration of a solid-state imaging device to which an embodiment ofthe present invention is applied, such as a CMOS image sensor forexample. As shown in FIG. 1, the CMOS image sensor 10 is of aconfiguration having a pixel array unit 11 formed on an unshownsemiconductor substrate (chip), and peripheral circuit portionsintegrated on the same semiconductor substrate as the pixel array unit11. The peripheral circuit portions include, for example, a verticaldriving unit 12, a column processing unit 13, a horizontal driving unit14, and a system control unit 15.

The CMOS image sensor 10 further has a signal processing unit 18 and adata storage unit 19. The signal processing unit 18 and data storageunit 19 may be realized by an external signal processing unit such as aDSP (Digital Signal Processor) provided on a substrate separate from theCMOS image sensor 10, or by software processing, and does not have to bemounted on the same substrate as the CMOS image sensor 10.

The pixel array unit 11 has unit pixels arrayed two-dimensionally inmatrix form. The unit pixels have photoelectric converters whichgenerate photo-generated charge (hereinafter may be referred to simplyas “charge”) of a charge amount corresponding to the amount of incidentlight, and accumulate the photo-generated charge internally. Thespecific configuration of unit pixels will be described later.

The pixel array unit 11 further has pixel driving lines 16 formedhorizontally in the drawing (in the direction of array of pixel rows)for each row of the matrix-form pixel array, and vertical signal lines17 formed vertically in the drawing (in the direction of array of pixelcolumns) for each column. While one pixel driving line 16 for each rowis shown in FIG. 1, the arrangement is not restricted to one. The otherend of the pixel driving lines 16 are connected to output terminals ofthe vertical driving unit 12 corresponding to each row.

The vertical driving unit 12 is configured of a shift register oraddress decoder or the like, and is a pixel driving unit for driving thepixels of the pixel array unit 11 either simultaneously for all pixels,or in increments of rows. The specific configuration of the verticaldriving unit 12 will not be described here, but generally is of aconfiguration having the two systems of a readout scanning system and asweep-out scanning system.

The readout scanning system selects and scans the unit pixels of thepixel array unit 11 in order of rows, to read out signals from the unitpixels. The sweep-out scanning system performs sweep-out scanning on thereadout rows to be subjected to readout scanning by the readout scanningsystem, by a timing advanced as to the readout scanning by an amount oftime equivalent to the shutter speed.

Due to the sweep-out scanning performed by the sweep-out scanningsystem, unnecessary charges are swept out from the photoelectricconverters of the unit pixels of the readout row (i.e., reset). Thesweeping out of the unnecessary charges (resetting) performed by thesweep-out scanning system realizes a so-called electronic shutteroperation. Note that the term “electronic shutter operation” as usedhere refers to the operation of discarding the photo-generated chargesin the photoelectric converters and starting a new exposure (startingaccumulation of photo-generated charges).

Signals read out by the readout operation performed by the readoutscanning system correspond to the amount of light input after theimmediately previous readout operation or electronic shutter operation.The period between the readout timing at which the immediately previousreadout operation was performed or the sweep-out timing at which theelectronic shutter operation was performed, to the readout timing atwhich the readout operation is performed this time, is the accumulationtime of photo-generated charges at the unit pixels (exposure time).

The signals output from each of the unit pixels in the pixel rowselected and scanned by the vertical driving unit 12 are supplied to thecolumn processing unit 13 via the vertical signal lines 17. The columnprocessing unit 13 performs predetermined signal processing on signalsoutput from each of the unit pixels of the selected row via the verticalsignal lines 17, and temporarily holds the pixel signals after thesignal processing, for each of the pixel rows of the pixel array unit11.

Specifically, the column processing unit 13 performs at least noiseremoval processing, CDS (Correlated Double Sampling) processing forexample, as signal processing. The fixed pattern noise unique to thepixels, such as reset noise, irregularities in the threshold values ofamplifying transistors, and so forth, is removed by the CDS processingperformed by the column processing unit 13. An arrangement may also bemade where in the column processing unit 13 is further provided with anAD (analog/digital) conversion function, for example, in addition to thenoise removal processing, so as to output the signal level in the formof digital signals.

The horizontal driving unit 14 is configured of a shift register oraddress decoder or the like, and selects unit pixels corresponding tothe pixel columns of the column processing unit 13, in order. Due to theselection and scanning performed by the horizontal driving unit 14,pixel signals subjected to signal processing at the column processingunit 13 are output in order.

The system control unit 15 is configured of a timing generator or thelike for generating various types of timing signals, and performsdriving control of the vertical driving unit 12, column processing unit13, horizontal driving unit 14, and so forth, based on the various typesof timing signals generated at the timing generator.

The signal processing unit 18 has at least an adding processingfunction, and performs various types of signal processing, such asadding processing and so forth, on the pixel signals output from thecolumn processing unit 13. The data storage unit 19 temporarily storesdata used in the signal processing performed by the signal processingunit 18.

Structure of Unit Pixel

Next, description will be made regarding a specific structure of a unitpixel 20. The unit pixel 20 is of a structure having a charge holdingregion (hereafter described as “memory unit”) for holding thephoto-generated charge transferred from the photoelectric converter,separate from the floating diffusion (capacitance). First through thirdspecific structural examples of the unit pixel 20 will now be described,with reference to FIGS. 2 through 7.

First Structural Example

FIG. 2 is a diagram illustrating the configuration of a unit pixel 20Aaccording to the first structural example. The unit pixel 20A accordingto the first structural example has a photodiode (PD) 21, for example,as a photoelectric converter. The photodiode 21 is an embeddedphotodiode formed by forming a p-type layer 33 on a p-type well layer 32formed on an n-type substrate 31 and embedding an n-type embedded layer34, for example.

The unit pixel 20A is of a configuration having a first transfer gate22, memory unit (MEM) 23, second transfer gate 24, and floatingdiffusion (FD) region 25, in addition to the embedded photodiode 21.Note that the memory unit 23 and floating diffusion region 25 areshielded from light.

The first transfer gate 22 transfers charges subjected to photoelectricconversion at the embedded photodiode 21 and accumulated therein, byapplication of a transfer pulse TRX to a gate electrode 22A. The memoryunit 23 is formed of an n-type embedded channel 35 formed below the gateelectrode 22A, and accumulates charge transferred from the embeddedphotodiode 21 by the first transfer gate 22. Forming the memory unit 23with the embedded channel 35 contributes to improved image quality,since occurrence of dark current at the Si—SiO2 interface can besuppressed.

The memory unit 23 can be modulated by disposing the gate electrode 22Aabove and applying the transfer pulse TRX to the gate electrode 22A.That is to say, applying the transfer pulse TRX to the gate electrode22A makes for a deeper potential for the memory unit 23. Accordingly,the saturation charge amount of the memory unit 23 can be increased overa case with no modulation.

The second transfer gate 24 transfers the charge accumulated at thememory unit 23 due to a transfer pulse TRG being applied to a gateelectrode 24A. The floating diffusion region 25 is an n-typecharge-voltage converter, which converts charge transferred from thememory unit 23 by the second transfer gate 24 into voltage.

The unit pixel 20A further has a reset transistor 26, an amplifyingtransistor 27, and a selecting transistor 28. N-channel MOS transistors,for example, are used here as the transistors 26 through 28, but itshould be noted that the combination of conductivity types of the resettransistor 26, amplifying transistor 27, and selecting transistor 28,illustrated here, is but one example, and that the invention is notrestricted to combinations thereof.

The reset transistor 26 is connected between power source VDB and thefloating diffusion region 25, and resets the floating diffusion region25 by a reset pulse RST being applied to the gate electrode thereof. Theamplifying transistor 27 has the drain electrode thereof connected topower source VDO, and the gate electrode connected to the floatingdiffusion region 25, so as to read out the voltage of the floatingdiffusion region 25.

The selecting transistor 28 has the drain electrode thereof connected tothe source electrode of the amplifying transistor 27 and the sourceelectrode thereof connected to the vertical signal line 17, for example,so as to select a unit pixel 20 regarding which the signal is to beready, upon application of a selecting pulse SEL to the gate electrodethereof. Note that an arrangement may be made wherein the selectingtransistor 28 is connected between the power source VDO and the drainelectrode of the second transfer gate 24.

It should be noted the part or all of the transistors 26 through 28 maybe omitted depending on the signal readout method, or may be sharedamong multiple pixels.

The unit pixel 20A further has a discharge portion 29 for dischargingthe accumulated charge in the embedded photodiode 21. The dischargeportion 29 discharges the charge of the embedded photodiode 21 to adrain portion 36 upon a control pulse ABG being applied to a gateelectrode 29A at the time of starting exposure. The discharge portion 29further acts to prevent the embedded photodiode 21 from overflowing dueto saturation during the readout period following exposure. Apredetermined voltage VDA is applied to the drain portion 36.

Note that with the unit pixel 20A according to the first structuralexample 1, a structure is employed wherein the gate electrode 22A of thefirst transfer gate 22 and memory unit 23 does not overlap the gateelectrode 24A of the second transfer gate 24. On the other hand, anarrangement may be made as shown in FIG. 3, where a gate electrode 22A′partially overlaps the gate electrode 24A, as a unit pixel 20A′ which isa modification of the first structural example.

Also, the first structural example employs a configuration wherein thedischarge portion 29 is employed to discharge the accumulated charge ofthe embedded photodiode 21 and to prevent overflow of the charge at theembedded photodiode 21, but an arrangement may be made as shown in FIG.4 wherein the transfer pulses TRX and TRS and the reset pulse RST areall in an active (“H” level in the present embodiment) state, therebyobtaining advantages equivalent to the discharge portion 29.

That is to say, having the first transfer gate 22, second transfer gate24, and reset transistor 26 all in an on (conducting) state enables thecharge of the embedded photodiode 21 to be discharged and alsooverflowing charge of the embedded photodiode 21 during the readoutperiod to be shunted to the substrate side. This arrangement allows thedischarge portion 29 to be omitted, which can contribute to reduced sizeof the unit pixel 20A.

Potential of Gate Electrode of Memory Unit 23

Now description will be made regarding the potential of the gateelectrode of the memory unit 23, which is the potential of the gateelectrode 22A of the first transfer gate 22 with this first structuralexample.

With the present embodiment, the potential of the gate electrode of thememory unit 23 serving as a charge holding region is set to a potentialwhereby at least one of the first transfer gate 22 and the secondtransfer gate 24, the first transfer gate 22 for example, is placed in apinning state when in a non-conducting state. More specifically, at thetime of placing one or both of the first transfer gate 22 and the secondtransfer gate 24 in a non-conducting state, the voltage applied to thegate electrodes 22A and 22B is set to a voltage such that a pinningstate is realized where carriers can be accumulated at the Si faceimmediately below the gate electrode.

In the event that the transistor forming the transfer gate is an n-typetransistor as with this example, at the time of placing the firsttransfer gate 22 in a non-conducting state, voltage is set such that thevoltage applied to the gate electrode 22A is a more negative potentialthan the ground GND as to the p-type well layer 32. Though not shown inthe drawings, the event that the transistor forming the transfer gate isa p-type transistor, the p-type well layer becomes an n-type well layer,and a voltage higher than the power source voltage VDD is set as to thisn-type well layer.

The reason for the voltage applied to the gate electrode 22A being setto a voltage such that a pinning state is realized where carriers can beaccumulated at the Si face immediately below the gate electrode, at thetime of placing the first transfer gate 22 in a non-conducting state, isas follows.

In the first structural example, if the potential of the gate electrode22A of the first transfer gate 22 is set to the same potential as to thep-type well layer 32 (e.g., 0 V), there is the possibility that carriersgenerated from crystal flaws at the Si surface may be accumulated at thememory unit 23, which may become dark current and deteriorate imagequality.

Accordingly, with the present embodiment, the OFF potential of the gateelectrode 22A formed on the memory unit 23 is set to a negativepotential, e.g., −2.0 V, as to the p-type well layer 32. Thus, with thepresent embodiment, holes are generated a the Si surface of the memoryunit during the period of holding a charge, which can be recombined withthe electrons generated at the Si surface, consequently allowingreduction in dark current.

Note that with the first structural example shown in FIG. 2, the gateelectrode 24A of the second transfer gate 24 exists at the edge of thememory unit 23, so setting this gate electrode 24A to a negativepotential as well enables dark current occurring at the edge of thememory unit 23 to be suppressed, in the same way.

FIG. 5 is a potential diagram in the Z-Z′ direction illustrated in FIG.5. As shown in FIG. 5, in the event that the transfer pulse TRX as tothe gate electrode 22A is Vg_a, 0 V for example, the Si surfacepotential φs is a positive value, and is depleted.

Accordingly, electrons generated due to crystal depletion at the Sisurface flow forward toward lower potential, and are accumulated at thememory unit 23 which is an n-type impurity diffusion region. In theevent that the transfer pulse TRX to the gate electrode 22A is Vg_b,sufficiently negative potential for example, the surface potential φs_bis a negative value, and holes are accumulated at the Si surface. Thus,electrodes generated at crystal flaws at the Si surface are recombinedwith accumulated holes, and are not accumulated at the memory unit 23.

FIG. 6 is a diagram illustrating an example of the relation between gatevoltage Vg and surface potential φs. In FIG. 6, the horizontal axisrepresents the gate voltage Vg, and the vertical axis represents thesurface potential φs.

As shown in FIG. 6, applying negative potential to the gate electrode22A (or 24A) causes the surface potential 0 to shift in the negativedirection, and from a certain value holes are accumulated and gatevoltage dependency of the surface potential almost disappears. That isto say, realizing a pinning state allows accumulation of holes at the Sisurface, yielding dark current reduction effects. Note that “sufficientnegative potential” as used above indicates this pinning state.

Second Structural Example

FIG. 7 is a diagram illustrating the configuration of a unit pixel 20Brelating to the second structure example, and components in FIG. 7 whichare equivalent to those in FIG. 2 are denoted with the same referencenumerals.

The unit pixel 20A according to the first structural example has astructure wherein the gate electrode 22A of the first transfer gate 22shares the electrode above the memory unit 23. On the other hand, theunit pixel 20B according to the second structural example has aconfiguration wherein the gate electrode 22A of the first transfer gate22 is separated from the electrode above the memory unit 23, a dedicatedelectrode 23A is provided above the memory unit 23, and the electrode23A is driven with a transfer pulse TRZ which is different from thetransfer pulse TRX.

Thus, pixel operations the same as with the case of the unit pixel 20Aaccording to the first structural example can be obtained by separatingthe gate electrode 22A of the first transfer gate 22 and the electrode23A of the memory unit 23, and driving the electrodes 22A and 23A withdifferent transfer pulses TRX and TRZ. Particularly, separating theelectrode 23A on the memory unit 23 from the gate electrode 22A allowsthe degree of modulation of the memory unit 23 by the transfer pulse TRZto be adjusted. Accordingly, the saturation charge amount of the memoryunit 23 can be freely set.

In the case of the second structural example as well, a configurationmay be made as with the first structural example, where the dischargeportion 29 is omitted and the transfer pulses TRX, TRZ, and TRS, andreset pulse RST, are all in an active state. Due to this configuration,advantages equivalent to those of the discharge portion 29 can beobtained, i.e., to enable the charge of the embedded photodiode 21 to bedischarged and also overflowing charge of the embedded photodiode 21during the readout period to be shunted to the substrate side.

With the second structural example, the potential of the gate electrode23A of the memory unit 23 serving as a charge holding region is set to apotential realizing a pining state, during the period in which the firsttransfer gate 22 and second transfer gate 24 are in a non-conductivestate.

Note that with the second structural example shown in FIG. 7, the gateelectrode 22A of the first transfer gate 22 and the gate electrode 24Aof the second transfer gate 24 exist at the edge of the memory unit 23,so setting these gate electrodes 22A and 24A to negative potentialenables dark current occurring at the edge of the memory unit 23 to besuppressed, as with the above-described first structural example.

Third Structural Example

FIG. 8 is a diagram illustrating the configuration of a unit pixel 20Crelating to the third structural example, and components in FIG. 6 whichare equivalent to those in FIG. 2 are denoted with the same referencenumerals.

The unit pixel 20A shown in the first structural example has aconfiguration wherein the memory unit 23 is formed of the embeddedchannel 35. On the other hand, the unit pixel 20C according to the thirdstructural example has the memory unit 23 formed not of the embeddedchannel 35 but of an embedded diffusion region 37.

A case wherein the memory unit 23 is formed of the embedded diffusionregion 37 can yield the same advantages as with a case where the memoryunit 23 is formed of the embedded channel 35. Specifically, an n-typediffusion region 37 is formed within the p-type well layer 32, and ap-type layer 38 is formed on the substrate face side, whereby darkcurrent occurring at the Si—SiO2 interface can be prevented from beingaccumulated in the embedded diffusion region 37 of the memory unit 23,which can contribute to improved image quality.

In the case of employing the third structure example, the impurityconcentration of the diffusion region 37 of the memory unit 23 ispreferably lower than the impurity concentration of the diffusion regionof the floating diffusion region 25. Setting the impurity concentrationthus raises the transfer efficient of charges from the memory unit 23 tothe floating diffusion region 25 by the second transfer gate 24.

Note that with the third structure example, the memory unit 23 is formedof the embedded diffusion region 37, but a structure may be employedwhere the memory unit 23 is not embedded, even through the dark currentoccurring at the memory unit 23 may increase.

In the case of the third structural example as well, a configuration maybe made as with the first structural example, where the dischargeportion 29 is omitted and the transfer pulses TRX, TRZ, and TRS, andreset pulse RST, are all in an active state. Due to this configuration,advantages equivalent to those of the discharge portion 29 can beobtained, i.e., to enable the charge of the embedded photodiode 21 to bedischarged and also overflowing charge of the embedded photodiode 21during the readout period to be shunted to the substrate side.

FIG. 9 illustrates the potential of each of the unit pixels 20A through20C according to the first through third structure examples. As can beclearly understood from the potential diagram of FIG. 9, the potentialis the same for each portion with the case of the first structureexample and second structure example, and particularly, the potential ofthe photodiode (PD) 21 and the potential of the memory unit (MEM) 23 isthe same. On the other hand, with the case of the third structureexample, the potential of the memory unit 23 is deeper than thepotential of the photodiode 21.

It should be noted that the conductivity types of the device structuresin the unit pixels 20A through 20C according to the first through thirdstructure examples are only an example, and the n-type and p-type may beinverted. Further, the conductivity type of the substrate 31 may also beeither n-type or p-type.

As described above, the unit pixel 20 (20A through 20C) according to thepresent embodiment have a memory unit 23 for holding (accumulating)photo-generated charge transferred from the photodiode 21, separatelyfrom the floating diffusion region 25, and further, have the firsttransfer gate 22 and second transfer gate 24. The first transfer gate 22transfers charge from the photodiode 21 to the memory unit 23. Thesecond transfer gate 24 transfers charge from the memory unit 23 to thefloating diffusion region 25.

The following description will be made with reference to an example of acase of using the unit pixel 20A according to the first structuralexample as the unit pixel 20 according to the present embodiment. Forsake of simplification, in the following description, the unit pixel 20Awill be referred to simply as “unit pixel 20”.

Feature Portion of Embodiment

With the CMOS image sensor 10 according to the present embodiment, allpixels start exposure at the same time, all pixels end exposure at thesame time, and the charge accumulated at the photodiode 21 istransferred to the shielded memory unit 23 and floating diffusion region25, thereby realizing global exposure. This global exposure realizesimaging with no distortion, from an exposure period matching for allpixels.

To realize this global exposure, with the unit pixel 20 according to thepresent embodiment, the first transfer gate 22 is driven as appropriateby a first voltage value which effects a conducting (on) state, a secondvoltage value which effects a non-conducting (off) state, and a thirdvoltage value which is between the first voltage value and the secondvoltage value, i.e., these three values. In the following description,the third voltage value will be referred to as “intermediate voltageVmid”.

Feature 1

With the present embodiment, the first transfer gate 22 is driven at theintermediate voltage Vmid one or more times during the imaging periodfrom starting exposure for all pixels to ending exposure thereof at thesame time (global exposure period), in a state with the second transfergate 24 off, as shown in FIG. 10A. This driving of the first transfergate 22 one or more times with the intermediate voltage Vmid is thefirst feature (feature 1).

Providing the memory unit 23 within the pixel means that the area of thephotodiode 21 decreases and accordingly the saturation charge Qs_pd ofthe photodiode 21 is smaller, but the reduction in saturation chargeQs_pd can be compensated for by the driving according to the feature 1.Specifically, by driving the first transfer gate 22 with theintermediate voltage Vmid before saturation of the photodiode 21, aphoto-generated charge which has occurred exceeding a certain level(predetermined charge) is transferred to the memory unit 23 as a signalcharge 1, and held in the memory unit 23. The saturation level of thephotodiode 21 is the level in a state wherein the second voltage valueis applied to the second transfer gate 24 and the second transfer gate24 is in an off state.

Applying the intermediate voltage Vmid means that the first transfergate 22, which transfers the photo-generated charge which has beengenerated exceeding a certain level to the memory unit 23 as the signalcharge 1, functions as an intermediate charge transfer unit. That is tosay, the first transfer gate 22 serving as the intermediate chargetransfer unit transfers charge which is generated by photoelectricconversion at the photodiode 21 and exceeds the predetermined chargedetermined by the voltage value of the intermediate voltage Vmid, to thememory unit 23 as the signal charge 1.

Feature 2

As a result of the driving of the feature 1, the photo-generated chargeQ generated by the photoelectric conversion during the exposure periodis accumulated in just the photodiode 21, or both the photodiode 21 andthe memory unit 23. Specifically, with a pixel wherein the incidentlight luminosity is luminosity of or greater than a predeterminedluminosity, i.e., a pixel where the light is strong, the charge isaccumulated in both the photodiode 21 and memory unit 23 as shown inFIG. 8B. Also, with a pixel wherein the incident light luminosity issmaller than the predetermined luminosity, i.e., a pixel where the lightis weak, the charge is accumulated in only the photodiode 21 as shown inFIG. 8C. This accumulating of the photo-generated charge Q in just thephotodiode 21, or both the photodiode 21 and the memory unit 23, is thesecond feature (feature 2).

Now, we will refer to the accumulated charge at the photodiode 21 asQpd, and the accumulated charge at the memory unit 23 (signal charge 1)as Qmem. With a pixel where the light is strong, the photo-generatedcharge Q is accumulated and held in both the photodiode 21 and thememory unit 23, so the saturation charge can be expanded to Qpd+Qmem.Also, with a pixel where the light is weak, the accumulated charge issmall and there is no charge transfer due to the first transfer gate 22driven at the intermediate voltage Vmid, so the generated charge is allheld in the photodiode 21 as the accumulated charge Qpd.

Feature 3

At the time of ending exposure, the second transfer gate 24 is turnedon, and the charge in the memory unit 23 is transferred to the floatingdiffusion region 25 (FIG. 8D). Subsequently, the first transfer gate 22is turned on, and the charge of the photodiode 21 (signal charge 2) istransferred to the memory unit 23 (FIG. 8E). As a result of executingthis operation for all pixels at the same time, the accumulated chargeis held by both the photodiode 21 and memory unit 23 during the readoutperiod. This holding of the accumulated charge by both the photodiode 21and memory unit 23 during the readout period is the third feature(feature 3).

Feature 4

In the readout operation, first, the signal level Vmem, which is theoutput signal 1 according to the charge amount of the accumulated chargeQmem held in the floating diffusion region 25, is read out. The readoutperiod for this signal level Vmem will be called a first D period. Next,the reset operation is performed by the reset transistor 26, and thereset level Vrst of the floating diffusion region 25 is read out. Thereadout period for this reset level Vrst will be called a P period.

Next, the charge Qpd is transferred from the memory unit 23 to thefloating diffusion region 25, and the signal level Vpd is read out asthe output signal 2 corresponding to the charge amount of the chargeQpd. The readout period for this signal level Vpd will be called asecond D period. This readout out of the signal levels Vmem and Vpd,which are the two output signals 1 and 2, and the reset level Vrst, isthe fourth feature (feature 4).

Feature 5

The signal Vmem and signal Vpd which have been read out in two times areeach subjected to noise reduction processing at the column processingunit 13 (see FIG. 1) for example, using the reset level Vrst.Subsequently, in the event that the signal level Vpd has exceeded thepredetermined threshold value, processing for adding the signal levelVmem and the signal level Vpd is performed at the downstream signalprocessing unit 18 (see FIG. 1). This adding of the post-noise-removalsignal level Vmem and signal level Vpd is the fifth feature (feature 5).

As can be seen from the features 3 through 5, a charge is held at boththe photodiode 21 and memory unit 23 during the readout period, and thetwo signals Vmem and Vpd and the reset level Vrst are read and added,whereby a broad dynamic range can be secured. Also, the sum of thesignal level Vpd and signal level Vmem is equivalent to the sum of theaccumulated charge Qpd and accumulated charge Qmem, and since Qpd+Qmemis the charge amount generated proportionately to the incident lightintensity E, linear input/output properties can be obtained.

The noise removal using the reset level Vrst is processing where kTCnoise is not removed with regard to the signal level Vmem read out inthe first D period, but is processing where kTC noise is removed withregard to the signal level Vpd read out in the second D period.

With pixels which have a small signal level and are affected by kTCnoise (i.e., pixels where the light is weak), all generated charge isaccumulated to the photodiode 21, and holding is performed at the memoryunit 23 formed of the embedded channel, whereby high S/N ratio can berealized due to removal of kTC noise. Not adding the noise component ofthe signal level Vmem at the time of lower output due to executingaddition processing following the noise removal processing only in caseswherein the signal level Vpd has exceeded the predetermined thresholdvalue also contributes to this high S/N ratio.

Also, irregularities in threshold values of the first transfer gate 22affect the luminosity level used for accumulation at the memory unit 23,but do not affect final input/output properties. For example, we willsay that the total charge amount Qall of a certain pixel was dividedinto the charge Qpd and charge Qmem and accumulated, while at anotherpixel, the amount transferred to the charge Qmem was smaller by ΔQth dueto threshold value irregularities. However, even in such a case, theaccumulation at the photodiode 21 is Qpd+ΔQth, while the accumulation atthe memory unit 23 is Qmem−ΔQth. Accordingly, executing the additionprocessing of feature 5 allows the total charge Qall to be ultimatelyobtained, since the accumulated charge deviation ΔQth of the photodiode21 is cancelled out.

Circuit Operations

Next, specific circuit operations of the CMOS image sensor 10 having theunit pixel 20 according to the present embodiment will be described.

Normal Global Exposure

To facilitate understanding, first, global exposure operations accordingto the related art (hereinafter referred to as “normal global exposureoperations”) will be described with reference to FIGS. 11 and 12. Notethat as described earlier, with normal global exposure operations, thesaturation charge Qs_pd is around half as compared to a pixel not havinga memory unit (MEM) if the pixel size is the same.

In the timing chart in FIG. 11 and the operation explanatory diagram inFIG. 12, (1) through (7) correspond to the (1) through (7) in thefollowing operational description.

Accumulation Phase

All pixels are exposed at the same time by the driving of (1) through(3).

(1) The discharge portion 29 of all pixels goes on at the same time, andthe charge in the photodiode 21 is discharged, thereby startingexposure.(2) The photo-generated charge generated at the photodiode 21 inaccordance with the incident light luminosity is accumulated at thephotodiode 21.(3) The first transfer gate 22 goes on for all pixels at the same time,the photo-generated charge Qpd accumulated in the photodiode 21 istransferred to the memory unit 23, and held therein.

Readout Phase

A signal readout operation of individual pixels or increments ofmultiple pixels is performed by the driving of (4) through (7). In thecase of this example, pixels are driven in increments of rows.

(4) The reset transistor 26 goes on, and the charge of the floatingdiffusion region 25 is discharged.(5) The reset level Vrst of the floating diffusion region 25 is read outvia the amplifying transistor 27 (P period).(6) The second transfer gate 24 goes on, and the charge Qpd held in thememory unit 23 is transferred to the floating diffusion region 25.(7) The signal level Vpd corresponding to the charge Qpd of the floatingdiffusion region 25 is read out via the amplifying transistor 27 (Dperiod).

Here, the signal level Vpd is the result of charge-to-voltage conversionobtained from the parasitic capacitance Cfd by the computationexpression

Vpd=Qpd/Cfd.

Also, noise included in the signal level Vpd can be removed byCorrelated Double Sampling (CDS) which obtains the difference betweenthe reset level Vrst and signal level Vpd. However, it should be notedthat the maximum charge Qpd_sat which can be handled is around half oreven less as compared with a pixel not having a memory unit 23 if thepixel size is the same.

Global Exposure According to Present Embodiment

Next, the circuit operations of the CMOS image sensor 10 according tothe present embodiment will be described. It is to be understood thatthe circuit operations described next are executed under drivingperformed by the vertical driving unit 12 which serves as a pixeldriving unit. FIG. 13 shows the driving timing of the presentembodiment.

A first point is that the first transfer gate 22 is driven at theintermediate voltage Vmid one or more times during the same imagingperiod for all pixels, in a state with the second transfer gate 24 off.Note that the intermediate voltage Vmid is a voltage between the voltageat which the first transfer gate 22 is on and the voltage at which thefirst transfer gate 22 is off.

A second point is that turning the second transfer gate 24 on at thetime of ending exposure transfers the accumulated charge Qmem in thememory unit 23 to the floating diffusion region 25, and turning thefirst transfer gate 22 on transfers the accumulated charge Qpd of thephotodiode 21 to the memory unit 23.

A third point is that during the readout period, signal readout of thecharge Qmem holed at the floating diffusion region 25, and signalreadout of the charge Qpd held at the memory unit 23, and readout of thereset level Vrst, are performed, with the accumulated charge being readout in two times.

In the timing chart in FIG. 13, the DH period is the signal readoutperiod of the charge Qmem, the DL period is the signal readout period ofthe charge Qpd, and the P period is the readout period of the resetlevel Vrst.

Accumulation Period

FIGS. 14 and 15 illustrate the operations from starting of exposure(start of accumulation) to end of exposure (end of accumulation). FIG.14 illustrates the way in which a photo-generated charge is transferredin a case wherein the incident light luminosity is of a luminosity equalto or greater than a predetermined luminosity (i.e., the incident lightis strong). FIG. 15 illustrates the way in which a photo-generatedcharge is transferred in a case wherein the incident light luminosity isof smaller than a predetermined luminosity (i.e., the incident light isweak).

In the timing chart in FIG. 13 and the operation explanatory diagrams inFIGS. 14 and 15, (1) through (10) correspond to the (1) through (10) inthe following operational description. The driving of (1) through (10)is performed for all pixels at the same time, though driving timedifference among pixels is permissible within a range in which imagingdistortion is tolerable. For example, an arrangement might be conceivedwhere the driving timing is intentionally shifted slightly for each, soas to suppress peak current and avoid voltage drop and the like, and soforth.

(1) The discharge portion 29 goes on, and the charge in the photodiode21 is discharged, thereby starting exposure.(2) The photo-generated charge generated at the photodiode 21 inaccordance with the incident light luminosity is accumulated at thephotodiode 21.(3) The first transfer gate 22 is driven at the intermediate voltageVmid, thereby transferring charge exceeding a certain accumulationamount at the photodiode 21 to the memory unit 23. That is to say, in acase wherein the accumulation amount in FIG. 15 is small, the entirecharge remains in the photodiode 21 and no charge transfer takes place.

The driving of (2)-(3) may be repeated as with (4)-(5) and (6)-(7).

(4), (6) Exposure and accumulation is continued.(5), (7) The first transfer gate 22 is driven at the intermediatevoltage Vmid, and a charge exceeding the certain accumulation amount atthe photodiode 21 is transferred to the memory unit 23.

The following operations are executed when exposure is ended.

(8) The reset transistor 26 goes on, and the charge in the floatingdiffusion region 25 is discharged (reset operation).(9) The second transfer gate 24 goes on, and the accumulated charge Qmemin the memory unit 23 is transferred to the floating diffusion region25. At this time, there is no accumulation at the memory unit 23 withdark pixels, so Qmem=0 with such pixels.(10) The first transfer gate 22 goes on, and the accumulated charge Qpdat the photodiode 21 is transferred to the memory unit 23.

The operations of the readout period are shown in FIG. 16. In the timingchart in FIG. 13 and the operation explanatory diagram in FIG. 16, (11)through (15) correspond to the (11) through (15) in the followingoperational description.

(11) The transfer operation at the time of ending exposure causes thecharge Qmem to be held at the floating diffusion region 25, and thecharge Qpd is held at the memory unit 23. As described above, Qmem=0with dark pixels.

The charge Qmem accumulated at the floating diffusion region 25 is readout via the amplifying transistor 27 as signal level Vmem.Charge-to-voltage conversion in accordance with Vpd=Qpd/Cfd is performedwith the parasitic capacitance Cfd at the floating diffusion region 25(DH period).

(12) The reset transistor 26 goes on, and the charge at the floatingdiffusion region 25 is discharged.(13) The reset level Vrst of the floating diffusion region 25 is readout (P period).(14) The second transfer gate 24 goes on, and the charge Qpd of thememory unit 23 is transferred to the floating diffusion region 25.(15) The charge Qpd at the floating diffusion region 25 is read out assignal level Vpd. Vpd=Qpd/Cfd holds (DL period).

In contrast to the normal global exposure timing as shown in FIG. 17A,the global exposure according to the present embodiment performs readoutof the signal Vpd of a certain pixel twice, in the DH period and the DLperiod, as shown in FIG. 17B. Also, there is the reset level Vrstreadout period (P period) between the DH period and the DL period.

Example of Driving Order

Generally, the floating diffusion region 25 has a greater dark currentas compared to a memory unit 23 formed of an embedded channel 35(structural examples 1 and 2) or embedded diffusion region 37(structural example 3). The charge Qmem, which is a part of theaccumulated charge, is held in the floating diffusion region 25 duringthe readout period, and accordingly is affected by dark current morethan in the holding period.

As shown in FIG. 17B, from ending of the accumulation period up to theDH period is the period of charge holding at the floating diffusionregion 25, so at the final readout row, the DH period, P period, and DLperiod has to be kept held for each row.

On the other hand, as shown in FIG. 18, an arrangement may be madewherein readout of the signal level Vmem in the DH period is allperformed first, with the signal level Vpd being read out later.According to this method, the charge holding period at the floatingdiffusion region 25 can be shortened, and effects of dark currentalleviated. It should be noted however, that with this case, one frameworth of data storage region (memory) has to be used to hold signalsread out in the DH period, so as to add the results read out in twotimes and obtain a final image.

Thus, with another driving example, the accumulated charge (signalcharge 1) Qmem is read out as signal level (output signal 1) Vmem, inincrements of single pixels or multiple pixels, following ending of theexposure period. Subsequently, the floating diffusion region 25 is resetand the reset level of the floating diffusion region 25 is read out as areset signal 1. This operation is sequentially performed on all of theunit pixels 20.

Subsequently, in increments of single pixels or multiple pixels, thefloating diffusion region 25 is reset and the reset level of thefloating diffusion region 25 is read out as a reset signal 2. Next, thesecond transfer gate 24 is placed in an on state and the accumulatedcharge (signal charge 2) Qpd is transferred to the floating diffusionregion 25, following which an operation is performed to read out theaccumulated charge Qpd as signal level (output signal 2) Vpd.

The noise removal processing at the column processing unit 13 in thecase of using this driving example is as follows. That is to say, thecolumn processing unit 13 performs noise removal processing of thesignal level Vmem which is the output signal 1, using the reset signal1. Next, the column processing unit 13 performs noise removal processingof the signal level Vpd which is the output signal 2, using the resetsignal 2.

Other Structure of Unit Pixel

Next, another configuration example of the unit pixel 20 will bedescribed as a fourth structural example.

Fourth Structural Example 4

FIG. 19 is a diagram illustrating the configuration of a unit pixel 20Daccording to the fourth structural example, with components in FIG. 17which are the same as those in FIG. 2 being denoted with the samereference numerals.

The unit pixel 20D according to structural example 4 has a structurewherein an overflow path 30 has been formed by providing an n-typeimpurity diffusion region 39 has been provided at the boundary portionbetween the photodiode 21 below the gate electrode 22A and the memoryunit 23.

In order to lower the potential of the impurity diffusion region 39 soas to form an overflow path 30, the impurity diffusion region 39 islightly doped with an n-type impurity to lower the p-type impurityconcentration, thereby forming a p− impurity diffusion region 39.Alternatively, in the case of doping the impurity diffusion region 39with a p-type impurity at the time of forming the potential barrier, ap− impurity diffusion region 39 can be formed by lowering theconcentration of doping.

Here, the unit pixel 20A according to the first structural example isused as a basis, but the unit pixel 20A according to a modificationthereof may be used as a basis instead.

As described above, with the unit pixel 20A according to the firststructural example (or modifications thereof in the same way), a featureis that the first transfer gate 22 is driven at the intermediate voltageVmid. Specifically, charge generated under low illuminance isaccumulated at the photodiode 21 with priority, a charge regarding whichsaturation occurs is accumulated in the memory unit 23 by driving of thefirst transfer gate 22 with the intermediate voltage Vmid. At the timeof ending exposure, transfer is performed from the memory unit 23 to thefloating diffusion region 25 and from the photodiode 21 to the memoryunit 23 for all pixels at the same time, and held, such that readout isperformed over two times.

On the other hand, with the unit pixel 20D according to the fourthstructural example, the overflow path 30, which has been formed at theboundary portion between the photodiode 21 and the memory unit 23, isused as the way for accumulating a charge generated under lowilluminance at the photodiode 21 with priority. FIG. 20A illustratesX-direction potential in FIG. 19, and FIG. 20B illustrates Z-directionpotential in FIG. 19.

As can be clearly seen from the X-direction potential diagram of FIG.20A, providing the n-type impurity diffusion region 39 at the boundaryportion between the photodiode 21 and the memory unit 23 lowers thepotential at this boundary portion. The portion where the potential hasbeen lowered becomes the overflow path 30. Charge generated at thephotodiode 21 and exceeded the potential of the overflow path 30 isautomatically leaked to the memory unit 23 and is accumulated at thememory unit 23. In other words, any charge generated within thepotential of the overflow path 30 is accumulated at the photodiode 21.

Now, the potential of the overflow path 30 has to be set lower than thepotential of the overflow path at the substrate side, as shown in theZ-direction potential diagram of FIG. 20B. The potential of the overflowpath 30 at this time is the potential which determines the charge amounttransferred to the memory unit 23 from the photodiode 21 as the signalcharge 1 upon the above-described intermediate voltage Vmid beingapplied to the gate electrode 22A.

Now, the overflow path 30 functions as an intermediate charge transferunit. That is to say, the overflow path 30 which serves as theintermediate charge transfer unit transfers a charge, which has beengenerated by photoelectric conversion at the photodiode 21 in theexposure period where all of multiple unit pixels perform imagingoperations, and exceeds the predetermined charge amount determined bythe potential of the overflow path 30, to the memory unit 23 as thesignal charge 1.

Note that in the example shown in FIG. 19, a structure is adoptedwherein the overflow path 30 has been formed by providing the p−impurity diffusion region 39. It should be noted, however, that as shownin FIG. 21, a structure may be used wherein the overflow path 30 isformed by providing an n− impurity diffusion region 39 instead of the p−impurity diffusion region 39.

As can be seen from FIGS. 19 and 21, with a structure wherein theimpurity concentration at the boundary between the photodiode (PD) 21serving as a photoreceptor and the memory is adjusted, and the overflowpath 30 has been provided between the photodiode (PD) 21 and memory unit23, the following advantages can be obtained other than occurrence ofdark current due to pinning.

FIG. 22 is a diagram illustrating the overflow path portion in FIG. 19in detail. A depletion layer 40 due to a p-n junction is formed nearbythe photodiode (PD) 21 and memory unit 23, with the depletion layer 40formed at the boundary of the photodiode (PD) 21 below the gateelectrode 22A of the first transfer gate 22 and the memory unit 23reaching to the Si surface.

Generally, if the depletion layer 40 is formed reaching to the Sisurface, the dark current due to crystal flaws at the Si surface isaccumulated at the photodiode (PD) 21 ort the memory unit, so in orderto avoid this, the gate electrode 22A is set to a negative potential torealize a pinning state, and a hole accumulation layer is formed at theSi surface.

In FIG. 19, the overflow path 30 with low potential is formed within thedepletion layer 40 between the photodiode (PD) 21 and the memory unit23, with a p− impurity diffusion region. In a state with no pinning, theoverflow path 30 also comes into contact with the Si surface, so at thetime that the charge overflows and is transferred to the memory unit 23,a phenomenon occurs wherein carriers disappear due to being trapped andrecombined at flaws at the Si surface.

Applying sufficient negative potential to the gate electrode 22A forms ahole accumulation layer at the Si surface side of the overflow path 30which raises the potential, so the overflow path 30 which has lowpotential shifts to a deeper position within the Si. Accordingly,carrier recombination due to crystal flaws can be prevented in chargetransfer to the memory 23 using overflow.

FIGS. 23A through 23C are plan views illustrating the planar structureof a unit pixel. FIG. 23A is a plan view of the unit pixel 20A accordingto the first structural example, and FIG. 23B is a plan view of the unitpixel 20D according to the fourth structural example. Here, the overflowpath 30 is formed over the entire region of the boundary portion betweenthe photodiode 21 and the memory unit 23. However, it should be notedthat this is only one example, and that an arrangement may be made withan overflow path 30′ at part of the boundary portion between thephotodiode 21 and the memory unit 23 as shown in FIG. 23C.

FIG. 24 is a timing chart illustrating the driving timing in the case ofusing the unit pixel 20D according to the fourth structural example,corresponding to FIG. 13. As can be clearly understood by comparing FIG.24 with FIG. 13, in the case of using the unit pixel 20D according tothe fourth structural example, the driving (3), (5), and (7) with theintermediate voltage Vmid in the accumulation period is done away with.The same two-time readout is performed following ending of exposure, asin FIG. 13.

Note that as described above, at the timing of driving the gateelectrode of the memory unit 23 at negative potential (pinning voltage),driving through a different voltage (e.g., 0V) in a transient manner canbe performed.

FIG. 25 is a timing chart illustrating an example of driving through adifferent voltage (e.g., 0V) in a transient manner at the time ofdriving at negative potential (pinning voltage), unlike that in FIG. 24.

In a case of setting a gate electrode, which is to be placed in anon-conducting state, to a voltage which realizes a pinning state (e.g.,negative potential), in the process of driving from the voltage Von inthe conducting state to the voltage Voff in the non-conducting state, avoltage Vtr between Von and Voff may be transiently passed through.

For example, in a case of setting the gate electrodes 22A and 24A of thefirst transfer gate 22 and second transfer gate 24 to a voltage whereina pinning state is realized when in a non-conducting state, driving suchas shown in FIG. 25 is performed. The transfer pulses TRX and TRG aretemporarily driven at Vtr at the time of driving from the voltage Von inthe conducting state to the voltage Voff in the non-conducting state,and then driven at Voff.

Voltage realizing a pinning state such as negative potential isgenerally generated by booster circuits or step-down circuits in manycases, and often has higher impedance as compared with normal powersources and grounds and accordingly tends to have inferior currentsupplying capabilities. Accordingly, driving from the voltage Von to thevoltage Voff directly places a great load on the booster circuit orstep-down circuit, and voltage convergence may lag.

To deal with this, the intermediate voltage Vtr is passed through todriving at the voltage Voff, thereby alleviating the load. The voltageVtr can be effectively obtained if a voltage between the voltages Vonand Voff, with the ground voltage (0 V) being used, for example.

Note that while the transfer pulses TRX and TRG have been exemplarilyillustrated in FIG. 25, this may be applied to any signal where thenon-conducting state is to be set to a pinning voltage.

FIGS. 26 and 27 are operation explanatory diagrams of during exposure inthe case of using the unit pixel 20D according to the fourth structuralexample, and correspond to FIGS. 14 and 15. FIG. 26 is an operationexplanatory diagram of during exposure in the case that the incidentlight is strong, and FIG. 27 is an operation explanatory diagram ofduring exposure in the case that the incident light is weak. As can beclearly from FIGS. 26 and 27, in the case of using the unit pixel 20Daccording to the fourth structural example, the driving of the firsttransfer gate 22 with the intermediate voltage Vmid is done away with.Instead, in the event that the charge generated at the photodiode 21exceeds the potential of the overflow path 30, this is transferred tothe memory unit 23.

FIGS. 28A and 28B are diagrams illustrating other driving timing ofglobal exposure in the case of using the unit pixel 20D according to thefourth structural example, and correspond to FIGS. 17B and 18. As can beclearly understood by comparing FIGS. 28A and 28B with FIGS. 17B and 18,in the case of using the unit pixel 20D according to the fourthstructural example, the driving with the intermediate voltage Vmid inthe accumulation period is done away with. The readout period ofP-phase/D-phase is the same as in FIGS. 17B and 18.

Now, while description has been made here that, in the case of using theunit pixel 20D according to the fourth structural example, driving withthe intermediate voltage Vmid is not applied and charge exceeding thepotential of the overflow path 30 (predetermined charge amount) istransferred to the memory unit 23 as the signal charge 1, but is notrestricted to this arrangement. That is to say, an arrangement may bemade wherein driving with the intermediate voltage Vmid is usedconjunctively, such that a charge which exceeds a predetermined chargeamount determined with the intermediate voltage Vmid and the potentialof the overflow path 30 is transferred to the memory unit 23 as thesignal charge 1.

Charge Accumulation

Next, charge accumulation at the photodiode 21 and the memory unit 23will be described, with comparison between a case of normal globalexposure and a case of global exposure according to the presentembodiment.

Normal Global Exposure

FIGS. 29A through 29C illustrate charge accumulation with normal globalexposure. The horizontal axis represent the amount of time from startingof exposure to ending of exposure, and the vertical axis is theaccumulated charge.

FIG. 29A illustrates charge accumulation at the photodiode 21. L1represents a case wherein the incident light is weak, and a chargeQch_all1 is generated during the exposure period. L2 represents a casewherein the incident light is strong, and a charge Qch_all2 whichexceeds the saturation charge amount Qpd_sat is generated at thephotodiode 21 during the exposure period.

FIG. 29B illustrates the accumulated charge at the memory unit 23 for L1which is the case that the incident light is weak. The accumulatedcharge Qpd of the photodiode 21 at the time of ending exposure isQch_all1, so the total charge Qch_all1 is transferred by charge transferby the first transfer gate 22. On the other hand, the accumulate chargeat the photodiode 21 is Qch_all1=0.

FIG. 29C illustrates the accumulated charge at the memory unit 23 for L2which is the case that the incident light is strong. The accumulatedcharge reaches the maximum charge amount (saturation charge amount)Qpd_sat of the photodiode 21 during exposure and saturates. Accordingly,the charge Qpd_sat is accumulated at the photodiode 21 at the time ofending exposure, so this charge Qpd_sat is transferred to the memoryunit 23 by the first transfer gate 22. The generated charge Qch_all2 isunobtainable due to saturation.

As can be clearly understood from the description of charge accumulationwith normal global exposure, with normal operations, the maximum chargeamount which can be obtained by photoelectric conversion at the unitpixel 20 is Qpd_sat which is the saturation charge amount of thephotodiode 21.

Global Exposure According to the Present Embodiment

FIGS. 30A through 30C illustrate charge accumulation with globalexposure according to the present embodiment. The horizontal axisrepresent the amount of time from starting of exposure to ending ofexposure, and the vertical axis is the accumulated charge.

FIG. 30A illustrates charge accumulation at the photodiode 21 for L1which represents a case wherein the incident light is weak, and L2 whichrepresents a case wherein the incident light is strong. FIGS. 30B and30C illustrate the accumulated charge at the memory unit 23 for L1 andL2, respectively. Also, (1) through (10) correspond to the drivingtiming (1) through (10) shown in the timing chart in FIG. 13.

Upon the first transfer gate 22 being driven with the intermediatevoltage Vmid at driving timings (3), (5), and (7), charge exceeding thecharge Qmid corresponding to the intermediate voltage Vmid istransferred to the memory unit 23. In the event that the accumulatedcharge at the photodiode 21 does not exceed the charge Qmid, the chargeremains in the photodiode 21.

With the example of L1, the incident light is weak and the accumulatedcharge is small, so the charge Qmid is not exceeded at the drivingtimings (3), (5), and (7), and there is no transfer by the firsttransfer gate 22. The total charge Qch_all1 is accumulated at thephotodiode 21, transferred to the memory unit 23 at the time of end ofexposure, and is as follows.

Qpd=Qch _(—) all1  (1)

Qmem=0  (2)

With the example of L2, the incident light is strong and the accumulatedcharge is great, so the charge Qmid is exceeded at the driving timings(3), (5), and (7). Here, an example is given where the charge Qmid isnot exceeded at the driving timing (3) but at the driving timings (5)and (7).

If we say that Qch_all2 represents the total charge to be generated inaccordance with the incident light during the exposure period, andcharges generated in each of four exposure periods divided by thedriving timings (3), (5), and (7) are represented by Qch1, Qch2, Qch3,and Qch4,

Qch _(—) all2=Qch1+Qch2+Qch3+Qch4  (3)

holds.

At the driving timing (3), the accumulated charge Qpd2_(—)1 at thephotodiode 21 is

Qpd2_(—)1=Qch1  (4)

and in the event of being lower than the charge Qmid, there is no chargetransfer to the memory unit 23. In the event of exceeding the chargeQmid, the charge Qtx1 is zero. At this time,

Qpd2_(—)1+Qtx1=Qch1  (5)

holds.

At the driving timing (5), the accumulated charge Qpd2_(—)2 at thephotodiode 21 is

Qpd2_(—)2=(Qpd2_(—)1−Qtx1)+Qch2  (6)

and in the case of exceeding the charge Qmid, charge transfer to thememory unit 23 occurs. The charge Qtx2 to be transferred to the memoryunit 23 is

Qtx2=Qpd2_(—)2−Qmid  (7)

at this time.

At the driving timing (7), the accumulated charge Qpd2_(—)3 at thephotodiode 21 is

Qpd2_(—)3=(Qpd2_(—)2−Qtx2)+Qch3  (8)

and in the case of exceeding the charge Qmid, charge transfer to thememory unit 23 occurs. The charge Qtx3 to be transferred to the memoryunit 23 is

Qtx3=Qpd2_(—)3−Qmid  (9)

at this time.

Exposure further continues, and the accumulated charge Qpd2_(—)4 at thephotodiode 21 at the time of ending exposure is

Qpd2_(—)4=(Qpd2_(—)3−Qtx3)+Qch4  (10).

Also, the charge Qmem accumulated at the memory unit 23 by chargetransfer under driving by the intermediate voltage Vmid is

Qmem=Qtx1+Qtx2+Qtx3  (11)

and is transferred to the floating diffusion region 25 immediately priorto ending of the exposure, and is held.

With the accumulated charge at the photodiode 21 at the time of endingexposure as Qpd,

Qpd=Qpd2_(—)4  (12)

holds. The accumulated charge at the photodiode 21 Qpd (Qpd2_(—)4) istransferred to the memory unit 23 by the first transfer gate 22 and isheld at the memory unit 23.

The charge Qmem held in the floating diffusion region 25 and the Qpdhold in the memory unit 23 due to the exposure ending and chargetransfer being performed are each read out and subjected to addingprocessing at the downstream signal processing unit 18 (see FIG. 1),thereby obtaining a signal level corresponding to the total generatedcharge Qch_all2

Expression (11) and (12) yield

Qpd+Qmem=Qpd2_(—)4+(Qtx1+Qtx2+Qtx3)

Expression (10) yields

=Qpd2_(—)3+Qch4+Qtx1+Qtx2

Expression (8) yields

=Qpd2_(—)2+Qch3+Qch4+Qtx1

Expression (6) yields

=Qpd2_(—)1+Qch2+Qch3+Qch4

Expression (4) yields

=Qch1+Qch2+Qch3+Qch4

and Expression (3) yields

=Qch _(—) all2  (13).

It can be understood from the above that the total generated chargeQch_all2 generated by photoelectric conversion at the unit pixel 20 canbe held and read out by reading out and adding each of the charges Qmemand Qpd.

The charge amount of the total generated charge Qch_all2 is linearlyproportionate to the intensity of the incident light, so it can beunderstood that image acquisition can be performed with linear responseproperties.

The adding processing at this time is performed at the signal processingunit 18 shown in FIG. 1. That is to say, the signal processing unit 18performs adding processing of the signal levels Vmem and Vpdcorresponding to the charges Qmem and Qpd read out divided in the DHperiod and DL period. At the time of this adding processing, the datastorage unit 19 shown in FIG. 1 temporarily stores the signal level Vmemcorresponding to the charge Qmem read out in the DH period.

Note that the adding processing at the signal processing unit 18 is notrestricted to adding processing of the signal levels Vmem and Vpdfollowing noise removal at the column processing unit 13. That is tosay, in a case of a configuration wherein the signal levels Vmem and Vpdare not to be subjected to the noise removal processing, addingprocessing is performed on the signal levels Vmem and Vpd output fromthe unit pixel 20.

A point to take note of in Expression (14) is that there is no effect ofthe holding charge Qmid of the photodiode 21 at the time of driving withthe intermediate voltage Vmid over the entire process up to obtainingthe total generated charge Qch_all2 from the charge Qmem and charge Qpd.This means that Qch_all2 can be obtained from Qmem+Qpd even in the eventthat the holding charge of the photodiode 21 is Qmid+ΔQmid due toirregularities in transistor threshold values among the pixels.

Let us consider a case, for example, wherein the holding charge Qmid ofthe photodiode 21 is Qmid+ΔQmid in Expressions (9) and (10). Here, thecharge Qtx3 in Expression (9) is

Qtx3=Qpd2_(—)3−(Qmid+ΔQmid)  (14)

and the value transferred to the memory unit 23 is reduced by ΔQmid.

On the other hand, the accumulated charge Qpd2_(—)4 in Expression (10)is

$\begin{matrix}\begin{matrix}{{{Qpd}\; 2\_ 4} = {\left( {{{Qpd}\; 2\_ 3} - {{Qtx}\; 3}} \right) + {{Qch}\; 4}}} \\{= {\left( {{Qmid} + {\Delta \; {Qmid}}} \right) + {{Qch}\; 4}}}\end{matrix} & (15)\end{matrix}$

so the accumulated charge at the photodiode 21 increases by ΔQmid.

The accumulated charge of the photodiode 21 and the accumulated chargeof the memory unit 23 are added to obtain the total generated chargeQch_all2.

Accordingly, the increase/decrease in accumulated charge at thephotodiode 21 is offset, meaning that irregularities in Qmid due tothreshold value irregularities do not affect the total photo-generatedcharge Qch_all2. Also, no effect of irregularities in Qmid means that,in other words, the intermediate voltage Vmid supplied to the firsttransfer gate 22 multiple times may be of different voltages each time.

FIGS. 31A through 31D illustrate input/output properties of the unitpixel 20. FIG. 31A illustrates the relation between the incident lightluminosity and the output from readout of the accumulated charge Qmem atthe DH period. Charge transfer from the photodiode 21 to the memory unit23 due to driving with the intermediate voltage Vmid is not output untila certain luminosity level E0, since this charge transfer occurs due tothe luminosity level E0 being exceeded and the accumulated charge in thephotodiode 21 exceeding Qmid.

FIG. 31B illustrates the relation between incident light luminosity andthe output from readout of the accumulated charge Qpd at the DH period.The luminosity level E0 is the luminosity level at which charge transferdue to transfer at the intermediate voltage Vmid begins to occur, andthe generated photo-generated charge at the luminosity level E0 isQpd_lin.

FIG. 31C illustrates the output of the added properties of FIGS. 31A and31B, i.e., the input/output properties of Qpd+Qmem. Charge can beaccumulated and held up to the sum of the maximum charge amounts Qpd_satand Qmem_sat of the photodiode 21 and memory unit 23. Imaging can beperformed up to the incident luminosity Emax which is equivalent toQpd_sat+Qmem_sat.

FIG. 31D illustrates input/output properties with normal global exposurenot using driving with the intermediate voltage Vmid. The maximum chargeamount Qpd_sat of the photodiode 21 is the maximum of accumulation andholding, so the dynamic range is low.

An arrangement is preferred wherein addition of the accumulated chargeQpd and the accumulated charge Qmem are not simplistically added, butrather where these are added only in the event that the accumulatedcharge Qpd exceeds the predetermined threshold value Qpd_th, andotherwise just the accumulated charge Qpd is output.

Qout=Qpd(Qpd≧Qpd _(—) th)

Qout=Qpd+Qmem(Qpd<Qpd _(—) th)

Now, the threshold value Qpd_th is a value smaller than the generatedphoto-generated charge Qpd_lin at the incident light luminosity E0,which is shown in FIGS. 31B and 31C. That is to say, the threshold valueQpd_th use for performing adding processing is a value below the signallevel Vpd serving as the output signal 2 which is equivalent to theminimum incident light luminosity E0 at which the signal level Vmemserving as the output signal 1 generates a significant output level.

In the event that the accumulated charge Qpd is smaller than thegenerated photo-generated charge Qpd_lin at incident light luminosityE0, the accumulated charge Qmem is signal output 0, so addition isunnecessary. This avoids needless noise component from being added whenreading out, and accordingly a high S/N ratio can be obtained alow-illuminance regions.

Removal of Reset Noise

The accumulated charge Qpd and the accumulated charge Qmem are subjectedto charge-to-voltage conversion into the signal level Vpd and signallevel Vmem at the floating diffusion region 25, and are read out via theamplifying transistor 27. At this time, signal actually being read outis a level where the signal level Vpd and signal level Vmem have beenadded as offset to the reset level Vrst.

The signal level Vsig_dh read out in the DH period is the reset levelVrst1 and signal level Vmem at the time of discharging the charge of thephotodiode 21 at the time of ending exposure.

Vsig _(—) dh=Vmem+Vrst1

Here, the reset level Vrst1 includes a fixed component Vrst_fpn such asoffset value, and a random component Vrst1_rn. The fixed componentVrst_fpn is threshold irregularities at the amplifying transistor 27 andload transistor (not shown) and so forth. The random component Vrst1_rnis kTC noise at the time of reset operation and so forth.

Vrst1=Vrst _(—) fpn+Vrst1_(—) rn

In the P period, a reset operation of the floating diffusion region 25is performed after the DH period, so the reset level Vrst becomes Vrst2,and this reset level Vrst2 is read out. This Vrst2 also includes a fixedcomponent and a random component. The fixed component is Vrst_fpn, thesame as with the reset level Vrst1, and the random component isVrst2_rn.

Vrst2=Vrst _(—) fpn+Vrst2_(—) rn

The signal level Vsig_dl read out at the DL period is

Vsig _(—) dl=Vpd+Vrst2.

The reset level Vrst2 read out in the P period by the noise removalprocessing at the column processing unit 13 (see FIG. 1) for example isremoved, so the DH period output Vout_dh and the DL period outputVout_dl are as follows.

$\begin{matrix}{{Vout\_ dh} = {{Vout\_ dh} - {{Vrst}\; 2}}} \\{= {{Vmen} + \left( {{{Vrst}\; 1{\_ rn}} - {{Vrst}\; 2{\_ rn}}} \right)}}\end{matrix}$ $\begin{matrix}{{Vout\_ dl} = {{Vsig\_ dl} - {{Vrst}\; 2}}} \\{= {Vpd}}\end{matrix}$

The charge Qpd accumulated in the photodiode 21 can be read out with thereset noise removed in a precise manner. The charge Qmem accumulated atthe memory unit 23 has the fixed component of the reset noise removed,but the random component (kTC noise, etc.) remains.

However, in the event that the incident light luminosity is high andgreat charges are generated, random noise generally is dominated byoptical shot noise, and the effects of reset noise and the like areextremely small. This is due to a physical phenomenon wherein randomnoise proportionate to the square root of the generated charge occurs.For example, if 10,000e-charges have been generated, 100e-rms of randomnoise will join as optical shot noise. On the other hand, random noiseoriginating in circuits is often around several e-rms, and hardlyaffects the image quality at all.

On the other hand, in the event that the incident light luminosity islow and few charges are generated, the optical shot noise itself issmall, so the effects of reset noise and the like are dominant, leadingto deterioration in image quality.

With the present embodiment, in the event that the incident lightluminosity is lower than a predetermined luminosity and few charges aregenerated, the charge is accumulated only in the photodiode 21, soremoval of reset noise can be performed in a precise manner, anddeterioration of image quality does not occur. Only in the event thatthe incident light luminosity is the predetermined luminosity or higherand great charges are generated, does accumulation of charge occur atthe memory unit 23, which is read out as accumulated charge Qmem, sothere is almost no deterioration of the image due to the above reason,and excellent imaging can be performed.

Advantages of the Present Embodiment

With the CMOS image sensor 10 according to the present embodimentdescribed above, imaging with no distortion can be realized even incases wherein the subject is moving and so forth, by arranging for allpixels to have the same exposure period (global exposure). In addition,a structure is employed wherein the unit pixel 20A has, in addition tothe floating diffusion region 25, a memory unit 23 capable ofaccumulating and holding signal charges, whereby the followingadvantages can be obtained.

In the event that the incident light luminosity is low illuminance belowa predetermined luminosity, there are few charges from photoelectricconversion, so the charge is stored only in the photodiode 21, and kTCnoise can be removed in the noise removal processing performed on thesignal level read out from the charge. Accordingly, a high S/N ratio canbe ensured since noise reduction processing including kTC noise as wellcan be realized.

In the event that the incident light luminosity is high illuminance ator above the predetermined luminosity, the charge generated byphotoelectric conversion is accumulated and held at both the photodiode21 and memory unit 23, so the saturation charge can be raised. Thecharge held in the photodiode 21 and memory unit 23 is read out andsignal levels Vpd and Vmem corresponding to the charge amounts thereofare added, whereby a wide dynamic range can be ensured.

While irregularities of the threshold value of the first transfer gate22 which is one of the pixel transistors affects the luminosity levelusing accumulation at the memory unit 23, this does not affect the finalinput/output properties, as described above. Accordingly, noise due toirregularities in threshold values of pixel transistors among the pixelscan be reduced, whereby the image quality of the taken image can beimproved.

The sum of the two signal levels Vpd and Vmem, corresponding to theaccumulated charges Qpd and Qmem of the photodiode 21 and memory unit 23are equivalent to the accumulated charge Qpd and accumulated chargeQmem, and the sum Qpd+Qmem is a charge generated proportionate to theincident light intensity E. Accordingly, linear response input/outputproperties are exhibited, so there is no problem with signal processingsuch as with color images.

Incidentally, in the event that the input/output properties are notlinear response, such as logarithmic response for example, this isunsuitable for signal processing such as with color images. For example,in the event that the RGB ratio of the illumination is 1:2:1, whitebalance is adjusted by acquiring the RGB ratio or all or part of theimaging face, and R and B are doubled. However, with logarithmicresponse, even of the RGB ratio of the illumination is constant, the RGBratio charges depending on the luminosity, so acquisition of RGB ratiois difficult. Further, even if this can be acquired, nonlinearadjustment has to be employed. Different input/output properties foreach pixel due to threshold irregularities makes the signal processingeven more difficult.

Even Higher Dynamic Range

With the operation description so far, the maximum charge amount isenlarged to Qpd_sat+Qmem_sat, whereby all generated charges are acquiredas signals, and a dynamic range is secured up to an incident lightluminosity equivalent to Qpd_sat+Qmem_sat.

Described below is a driving example wherein a part of the generatedcharge is discarded, and signals obtained by two types of exposure timesare output, to further expand the dynamic range. The basic principle ofdynamic range widening described here are applications of a principledescribed in Japanese Patent Application Nos. 2006-280959 and2006-124699, filed by the Present Assignee, to the structure of anembodiment of the present invention for realizing global exposure.

FIG. 32 illustrates a driving example for dynamic range widening. Thearrangement shown here is that shown in FIG. 13 according to anearlier-described diving example according to the present embodiment,but with discharge driving (16) of the memory unit 23 having been added.

With this driving example, driving with the intermediate voltage Vmid isperformed to the first transfer gate 22 twice or more during theexposure period which is common for all pixels. The second transfer gate24 is turned on at a period between the final intermediate voltagedriving (first intermediate voltage driving) and the intermediatevoltage driving preceding that intermediate voltage driving (secondintermediate voltage driving). The reset transistor 26 may or may not bego on at the same time. Also, each intermediate voltage Vmid in theintermediate voltage driving performed two times or more is preferablyof the same voltage value.

FIG. 33 illustrates the operations of dynamic range widening. Adding thedischarge driving (16) of the memory unit 23 discharges the chargeaccumulated in the memory unit 23 so far, and accordingly only thecharge transferred at the final intermediate voltage driving (firstintermediate voltage driving) becomes the accumulated charge Qmem. Thisaccumulated charge Qmem is such that in the event that the intermediatevoltages driving the first transfer gate 22 are equal, the chargeamounts remaining at the photodiode 21 at the first intermediate voltagedriving (7) and the previous second intermediate voltage driving (5) areeach equal at Qmid. Even in the event that the value of Qmid may differdue to threshold irregularities among pixels, the residual charge amountof the driving (5) and driving (7) will still be equal at Qmid.

The charge accumulated at the photodiode 21 immediately prior to thefirst intermediate voltage driving (7) is a value which can be obtainedby adding the charge Qshort accumulated from the second intermediatevoltage driving (5) to the first intermediate voltage driving (7), whichis a second exposure period, being added to the residual charge Qmid ofthe second intermediate voltage driving (5). That is to say, the chargeQmid remains in the photodiode 21 due to the first intermediate voltagedriving (7), so the accumulated charge Qmem is

Qmem=(Qmid+Qshort)−Qmid.

With the total charge generated in the total exposure period Tlong,which is a first exposure period, as Qlong, the charge Qshort is

Qshort=(Tshort/Tlong)×Qlong.

That is to say, a value which is small by the exposure ratio which isthe ratio between the intermediate voltage driving period Tshort and thetotal exposure period Tlong is output as the charge Qshort.

Accordingly, the charge Qshort can acquire signals even for incidentlight luminosity exceeding the maximum charge amount Qmem_sat, sodynamic range widening of a multiple of the exposure ratio can berealized. That is to say, imaging can be performed for up to an incidentlight luminosity equivalent to (Tlong/Tshort)×Qmem_sat.

FIGS. 34A through 34C illustrate input/output properties for the dynamicrange widening operation. In FIGS. 34A through 34C, E0 is a luminositylevel where transfer at the first intermediate voltage driving starts,i.e., the luminosity where the total generated charge up to the firstintermediate voltage driving is Qmid. E1 is a luminosity level wheretransfer at the second intermediate voltage driving starts, i.e., theluminosity where the total generated charge up to the secondintermediate voltage driving is Qmid.

At a luminosity level of E0 or lower, the generated charge is allaccumulated at the photodiode 21, and output as accumulated charge Qpd.At this time, Qmem=0.

$\begin{matrix}{{Qout} = {{Qpd}\mspace{14mu} \left( {E < {E\; 0}} \right)}} \\{= {Qlong}}\end{matrix}$

At luminosity levels from E0 to E1, there is no transfer at the secondintermediate voltage driving, and at the first intermediate voltagedriving the accumulation in the second exposure period Tshort is addedand the portion which has exceeded Qmid is transferred to become Qmem.In this case, at the point of the discharging operation (16) of thememory unit 23, Qmem=0, so the entirety of the generated charge isaccumulated as charge Qpd and Qmem, and the total generated charge canbe obtained as output Qout from Qpd+Qmem.

$\begin{matrix}{{Qout} = {{Qpd} + {{Qmem}\mspace{14mu} \left( {{E\; 0} \leq E < {E\; 1}} \right)}}} \\{= {Qlong}}\end{matrix}$

At a luminosity level exceeding E1, transfer occurs in the secondintermediate voltage driving, and the charge transferred thereto isdiscarded in the discharging driving (16). Accordingly, the totalgenerated charge is not obtained with Qpd+Qmem. However, as describedabove, due to transfer occurring in the second intermediate voltagedriving, the residual charge in the photodiode 21 is Qmid. Further, dueto the residual charge being Qmid the same as with the firstintermediate voltage driving, the charge Qshort accumulated in thesecond exposure period Tshort is transferred to and held in the memoryunit 23.

In this case, the gain of the exposure ratio Tlong/Tshort is multipliedfor the output Qout, a signal equivalent to the total charge Qlong canbe obtained.

That is,

$\begin{matrix}{{Qout} = {\left( {{Tlong}/{Tshort}} \right) \times {Qmem}\mspace{14mu} \left( {E \geq {E\; 1}} \right)}} \\{= {\left( {{Tlong}/{Tshort}} \right) \times {Qshort}}} \\{= {Qlong}}\end{matrix}$

which is to say

$\begin{matrix}\begin{matrix}{{Qout} = {{Qpd}\mspace{14mu} \left( {E < {E\; 0}} \right)}} \\{= {{Qpd} + {{Qmem}\mspace{14mu} \left( {{E\; 0} \leq E < {E\; 1}} \right)}}} \\{= {\left( {{Tlong}/{Tshort}} \right) \times {Qmem}}}\end{matrix} & (16)\end{matrix}$

whereby the linear properties such as shown in FIG. 34C can be obtained.The dynamic range at this time can be expanded to an incident lightluminosity Emax which is equivalent to (Tlong/Tshort)×Qmem_sat.

Also, the same advantages can be obtained by actually executing theabove processing as follows.

Qout = Qpd  (Qpd < Qpd_th)Qout = MAX(Qpd + Qmem(Tlong/Tshort) × Qmem)  (Qpd ≥ Qpd_th)

Now, Qpd_th is an accumulated charge amount equivalent to an incidentlight luminosity smaller than E0. In E<E0, Qmem=0 holds, and may beadded, but noise included in the charge Qmem might be added to theoutput in regions where signals are particularly small, and to this endthe Qpd_th has been provided.

MAX (A, B) is a function where the greater of A and B is selected. InE0≦E<E1, the obtained charge Qmem is smaller than the charge Qshortaccumulated in the second exposure period, so Qpd+Qmem is selected.Also, in E≧E1, the charge is discharged by the second transfer driving,so Qpd+Qmem is smaller than Qlong. Accordingly, (Tlong/Tshort)×Qmem isselected, and is equivalent to Expression (16).

The above calculation expression does not have to have points E0 and E1set which strictly switch over at each pixels, to set a threshold valueQpd_th equivalent to a luminosity level sufficiently smaller than E0 issufficient, and accordingly practical processing can be executed.

Modification

While a configuration has been described in the above embodiment whereinthe data storage unit 19 is provided downstream from the columnprocessing unit 13 parallel to the signal processing unit 18, theembodiment is not restricted to this arrangement. For example, anarrangement may be made such as shown in FIG. 35, wherein the datastorage unit 19 is provided in parallel with the column processing unit13, and the data Ddh of the DH period and the data Ddl of the DL periodare read out at the same time by horizontal scanning by the horizontaldriving unit 14, with signal processing being performed at thedownstream signal processing unit 18.

Further, an arrangement may be made such as shown in FIG. 36, whereinthe column processing unit 13 is provided with an AD conversion functionfor AD conversion for each column or increments of multiple columns ofthe pixel array unit 11, and also the data storage unit 19 and signalprocessing unit 18 are provided in parallel as to the column processingunit 13, so that following analog or digital noise removal processing atthe signal processing unit 18, the processing of the data storage unit19 and signal processing unit 18 is performed for each column orincrements of multiple columns.

Also, while an example has been given in the above embodiment whereinapplication is made to a CMOS image sensor in which unit pixels, whichdetect signal charges corresponding to the luminous quantity of visiblelight as physical amounts, arrayed in a matrix form, but the presentinvention is not restricted to application to a CMOS image sensor, andis applicable to column type solid state imaging devices in generalwhich a column processing unit is provided for each pixel column of thepixel array unit.

Also, the present invention is applicable to not only solid stateimaging devices which detect distribution of incident light quality ofvisible light and perform imaging thereof as an image, but also to solidstate imaging devices which detect distribution of the input quantity ofinfrared rays, X-rays, particles, etc., and further, to solid stateimaging devices (physical quantity detecting devices) in general in abroad sense, such as fingerprint detecting sensors or the like whichdetect the distribution of pressure, capacitance, and other physicalquantities, and perform imaging thereof as an image.

Note that the solid state imaging device may be configured as asingle-chip device, or may be in a modular form wherein an imaging unit,signal processing unit, optical system, or the like, are packagedtogether to have imaging functions.

Further, note that the present invention is not restricted toapplication to solid state imaging devices, and is also applicable toelectronic equipment in general using a solid state imaging device foran image acquisition unit (photoelectric converter), such as imagingapparatuses such as digital still cameras and video cameras and thelike, portable terminal devices having imaging functions as withcellular phones, photocopiers using solid state imaging devices for theimage pickup unit, and so forth. The present invention may further beapplied to imaging apparatuses wherein the above modular state, i.e., acamera module, is provided to electronic equipment.

Application Example

FIG. 37 is a block diagram illustrating an example of a configuration ofelectronic equipment according to an embodiment of the presentinvention, such as an imaging apparatus for example. As shown in FIG.37, an imaging apparatus according to an embodiment of the presentinvention includes an optical system having a lens group 51 and thelike, an imaging device 52, a DSP circuit 53 which is a camera signalprocessing circuit, frame memory 54, a display device 55, a recordingdevice 56, an operating system 57, a power source system 58, and soforth, with the DSP circuit 53, frame memory 54, display device 55,recording device 56, operating system 57, and power supply system 58being connected mutually by a bus line 59.

The lens group 51 inputs incident light (image light) from a subject andimages this on the imaging face of the imaging device 52. The imagingdevice 52 converts the luminous quantity of the incident light aimed onthe imaging face by the lens group 51 into electric signals inincrements of pixels, and outputs as pixel signals. A solid stateimaging device such as the CMOS image sensor 10 according to theabove-described embodiment, i.e., a solid state imaging device which canrealize imaging with no distortion by performing global exposure, can beused for the imaging device 52.

The display device 55 is configured of a panel type display device suchas a liquid crystal display device, organic EL (electroluminescence)display device, or the like, and displays moving or still images imagedat the imaging device 52. The recording device 56 records the moving orstill images imaged at the imaging device 52 on a recording medium suchas a video tape, DVD (Digital Versatile Disc), or the like.

The operating system 57 gives operating commands regarding variousfunctions which the imaging apparatus has, under operations performed bythe user. The power supply system 58 supplies the DSP circuit 53, framememory 54, display device 55, recording device 56, and operating system57, with appropriate power for each, to serve as operating power supplythereof.

As described above, using the CMOS image sensor 10 in theabove-described embodiment as the imaging device 52 in an imagingapparatus such as a video camera, digital still camera, camera modulefor mobile devices such as cellular phones and the like, and so forth,enables high image quality to be obtained for imaged images, since noisedue to threshold irregularities of pixel transistors can be reduced anda high S/N ratio can be ensured with the CMOS image sensor 10.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A solid state imaging device comprising: (1) aunit pixel including (a) a photoelectric converter configured togenerate electrical charge in accordance with incident light quantityand accumulate said charge therein, (b) a first transfer gate configuredto transfer the charge accumulated in said photoelectric converter, (c)a charge holding region configured to hold the charge transferred fromsaid photoelectric converter by said first transfer gate, (d) a secondtransfer gate configured to transfer the charge held in said chargeholding region, and (e) a floating diffusion region configured to holdthe charge, transferred from said charge holding region by said secondtransfer gate, for being read out as a signal; (2) an intermediatecharge transfer unit configured to transfer, to said charge holdingregion, a charge which exceeds a predetermined charge amount, generatedat said photoelectric converter in an exposure period in which all ofsaid plurality of unit pixels perform imaging operations at the sametime, as a first signal charge; and (3) a pixel driving unit.
 2. Aelectronic equipment comprising a solid state imaging device, the solidstate imaging device including: (1) a unit pixel including (a) aphotoelectric converter configured to generate electrical charge inaccordance with incident light quantity and accumulate said chargetherein, (b) a first transfer gate configured to transfer the chargeaccumulated in said photoelectric converter, (c) a charge holding regionconfigured to hold the charge transferred from said photoelectricconverter by said first transfer gate, (d) a second transfer gateconfigured to transfer the charge held in said charge holding region,and (e) a floating diffusion region configured to hold the charge,transferred from said charge holding region by said second transfergate, for being read out as a signal; (2) an intermediate chargetransfer unit configured to transfer, to said charge holding region, acharge which exceeds a predetermined charge amount, generated at saidphotoelectric converter in an exposure period in which all of saidplurality of unit pixels perform imaging operations at the same time, asa first signal charge; and (3) a pixel driving unit.